The only way how to somehow convert BPN files to DWG, would be to recover the *.bpn backup using the ArchiCAD and export it to DWG format using the default Export/Save as.. function.
Convert ArchiCAD backup to AutoCAD drawing.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert bpn to dwg on Windows
One of the architecture CAD software programs
Convert from bpn file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.